1. Field of the Invention
The present invention relates to an imaging apparatus which has an image stabilizing function.
2. Description of the Related Art
An imaging apparatus typified by a still camera or a video camera has an optical image stabilizing system or an image sensor image stabilizing system as a system for correcting vibrations such as camera shakes applied to the apparatus from the outside.
These systems perform digital signal processing on a signal from a vibration detection sensor which detects a vibration degree, via an analog-to-digital (A/D) conversion unit, calculate a vibration correction amount to execute digital-to-analog (D/A) conversion, and then drive a correction unit for image stabilizing, i.e., a shift lens or an image sensor.
An angular speed sensor is often used for vibration degree detection. This angular speed sensor vibrates a vibration material such as a piezoelectric element at a constant frequency, and converts Coriolis force generated by a rotational motion component into a voltage to obtain an angular speed signal.
As an apparatus for performing A/D conversion, digital signal processing, or D/A conversion, a microcomputer is used which includes a filter for cutting off a plurality of predetermined frequencies and an integration filter. Non-recursive and recursive digital filters are available as such filters.
FIG. 9A is an overall block diagram of a non-recursive primary digital filter. As compared with the recursive digital filter, the non-recursive digital filter includes only a feed-forward unit. When an input value X[n] is obtained at current sampling, an input value of last sampling is X[n−1], which is an intermediate value in the non-recursive digital filter. In other words, in the non-recursive digital filter, a value after passage through a delay element Z−1 is an intermediate value.
FIG. 9B illustrates an operational expression when gains of the feed-forward unit are set to constants a and b. To configure a filter having desired characteristics, values and signs of the constants a and b are appropriately set. Setting these constants enables configuration of a digital high-pass filter or a digital low-pass filter. Secondary and higher-order digital filters are realized by increasing delay elements Z−1. The number of intermediate values increases according to an order.
FIG. 9C is an overall block diagram of a recursive primary digital filter. The recursive digital filter includes a feed-forward unit and a feedback unit. In the recursive digital filter, an intermediate value is a calculation result of the feedback unit. In this case, an intermediate value Z[n] as shown in FIG. 9C is obtained at current sampling. A value after passage through a delay element Z−1 indicates last sampling. The delay element determines a digital filter order.
FIG. 9D illustrates the feedback unit cut out from the recursive digital filter. An intermediate value Z[n] of current sampling is calculated from an input value X[n] of the current sampling and an intermediate value Z[n−1] of last sampling, where n denotes a sampling cycle.
FIG. 9E illustrates the feed-forward unit cut out from the recursive digital filter. An output value Y[n] of the current sampling is calculated from the intermediate value Z[n] of the current sampling and the intermediate value Z[n−1] of the last sampling, where n denotes a sampling cycle.
FIG. 9F illustrates an operational expression when gains of the feed-forward unit and the feedback unit are respectively set to constants a, b, and c. Secondary or higher order digital filters are realized by increasing delay elements Z−1. The number of intermediate values is increased according to the increased order.
The optical image stabilizing system corrects image vibrations on the image sensor (removes image vibrations from an image formed on the image sensor) by moving the shift lens which is a correction unit within a plane orthogonal to an optical axis, to a driving target position calculated by using a vibration correction amount. The image sensor image stabilizing system corrects image vibrations on the image sensor by moving the image sensor which is a correction unit within the plane orthogonal to the optical axis, to a driving target position calculated by using a vibration correction amount. The present invention described below can be applied to both systems, and thus a configuration of the optical image stabilizing system will be described below as a representative example.
In the imaging apparatus having the image stabilizing function of the aforementioned system, a shift lens drive unit is instructed to move by a vibration correction amount. When the shift lens that is a control target reaches a driving target position, a real position of the shift lens is obtained. Feedback control is performed to reduce a deviation between the driving target position and the real position to zero. As one example of a technique for performing such feedback control, Japanese Patent Application Laid-Open No. 7-199263 discusses a technique of returning a shift lens to a predetermined position within a driving range before exposure when a driving amount of the shift lens is large.
A technique of removing an offset may have a following configuration. The configuration includes a unit for differentially amplifying a signal from a sensor and a reference voltage, a unit for calculating an offset component from the amplified signal, and a unit for changing the reference voltage. An offset is calculated by microcomputer processing, and the reference voltage of an amplifier is accordingly changed to remove an offset component from the amplified signal. By applying this technique to a vibration correction, an offset component is calculated from a vibration amount of the amplified signal output from an angular speed sensor, and the reference voltage of the amplifier is changed according to the offset amount, whereby the offset component of the amplified vibration signal is removed.
However, this system necessitates a new hardware mechanism such as a D/A conversion unit for changing the reference voltage. In the system, the amplifier vibration signal stepwise fluctuates immediately after the reference voltage change of the amplifier, and vibrations are erroneously detected when actually no vibration is detected. When the shift lens is driven by using such erroneously detected vibration signal, a problem may occur that a shot image moves unnaturally or a vibrated image is captured due to the processing for reference voltage change depending on an exposure timing.